Lecture 7: PROTEINS IN ACTION - 1 Flashcards
What is the concentration of oxygen in saline solution limited to?
Approximately 0.2 mmol/L
What is the concentration of haemoglobin in blood?
Approximately 5 mmol/L
What does adding haemoglobin do?
Allows for delivery of much more oxygen
What is highly active tissue limited by?
Availability of oxygen
What is there strong evolutionary pressure for?
Efficient oxygen delivery
What type of protein is myoglobin?
Heme
What does myoglobin do?
Stores oxygen in muscles against bursts of high requirements
How much myoglobin does human muscle have?
0.5-0.7 mol/L which is enough for about 7 seconds of intense activity
What does the tissue depend on after the myoglobin store is exhausted?
Oxygen delivery from the lungs
What is the primary structure of myoglobin?
Approximately 150 amino acids
What is the secondary structure of myoglobin?
Eight alpha helices (A-H) and connecting loops (AB, BC, etc)
What is the tertiary structure of myoglobin?
Globin fold with a hydrophobic pocket
What does the heme bind to in the global protein in myoglobin?
His F8 (eight amino acid in helix F, which is a histidine)
What is the quaternary structure of myoglobin?
Monomeric (a single polypeptide chain)
What does the global fold in myoglobin do?
Provides a hydrophobic pocket (ValE11 and Phe CD7) to bind a heme group
What is heme?
A prosthetic group or cofactor
What is in the heme?
Four pyrrole rings linked together (a protoporphyrin) in a plane
What does iron have?
Six coordinate bonds - four to nitrogen atoms of the heme, one to a nitrogen atom of Histidine F8 of the globin, one to oxygen
What gives the heme a red colour?
Molecular electronic orbitals or protoporphyrin
What type of reaction is the binding of oxygen to iron?
Reversible
What doe spectroscopy do?
Quantify dissolved materials
What does higher concentration mean in spectroscopy?
Less transmitted light = higher absorbance
What does the beer-lambert law do?
Converts from absorbance to concentration
How are different wavelengths absorbed?
More or less effectively
What does the shape of the spectrum differ with?
Colour and chemical nature of the solute
What colour is protein?
Colourless (but has UV absorbance)
What does heme have?
Visible absorbance (and therefore colour) that differs between bright red oxyhemoglobin and dull red deoxyhaemoglobin
How is heme fe2+ attached to global protein?
By coordinate linkage to HisF8
Where is another His located?
In helix E (hisE7) on the opposite side of the heme and distorts binding of gas molecules to 6th coordination position on fe2+
What does hisE7 do?
Reduces the binding affinity of oxygen to myoglobin, making it easier to release oxygen to the muscle cell
What does lactate do to myoglobin?
Decreases its affinity to oxygen but does not bind where oxygen binds. Allosteric control
What does lactate cause to change in the curve of myoglobin?
Shifts the curve right
What does lactate build up in muscles promote?
Oxygen release from myoglobin, increasing oxygen availability for respiration
What does allosteric mean?
Without overlapping (not in the same place)
What does the availability of oxygen to cellular respiration depend on?
The partial pressure of oxygen in the local environment and the binding affinity of oxygen to haemoglobin
When is myoglobin saturated?
At low partial pressure of oxygen, only releasing oxygen to muscle cells when the cellular partial pressure of oxygen is very low
How is the oxygen binding of myoglobin shown?
In a hyperbolic curve
What does the oxygen binding of myoglobin suit?
Its function as a back up store of oxygen in muscle cells
What is the partial pressure of oxygen in lungs?
Approximately 100 torr
What is the partial pressure of oxygen in resting muscle?
Approximately 20 torr
What is the function of haemoglobin?
An oxygen transporter in blood and has much weaker binding affinity for oxygen than myoglobin
How is the oxygen binding of haemoglobin shown?
As a sigmoidal curve
What does haemoglobin in red blood cells in the blood need to be able to do?
Bind oxygen in the vicinity of the lungs and release oxygen in the vicinity of peripheral tissues
What did haemoglobin evolve to do?
Bind oxygen less tightly than myoglobin
What can haemoglobin do?
Change shape, “dumping” oxygen in peripheral tissues
What structure did haemoglobin evolve?
To be a tetramer - four global proteins associate together non-covalently
What does each global protein in haemoglobin contain?
One heme and each can bind one oxygen (1, 2, 3, or 4 oxygen can bind per one haemoglobin tetramer)
What is the structure of myoglobin?
Monomer
The structures of oxyhemoglobin and deoxyhaemoglobin are…
Similar but different
What does MWC in the concerted model stand for?
Monod, Wyman and Changeux
What conformations can subunits be in?
Low activity, tense (T) or high activity, relaxed (R)
What must all subunits be in?
The same state (T or R)
What does binding each successive substrate do?
Shift the equilibrium in favour of R
What stabilises the T form?
Inhibitors
What stabilises the R form?
Activators
What does KNF stand for in the sequential model?
Koshland, Nemethe and Filmer
What does only one substrate binding do in the sequential model?
Induces a T to R conformational change in only one subunits
What does the conformational change in the sequential model do?
Influences the neighbouring subunit (cooperativity), making them more likely to bind substrate. Many conformations possible
What does the sequential model explain?
Negative cooperativity
What does oxygen do in myoglobin and haemoglobin?
Binds to the iron of heme
What happens in myoglobin and haemoglobin to monitor oxygen binding?
A shift from dull to bright red
What happens to the affinity of oxygen in myoglobin and haemoglobin?
It is altered by molecules (such as lactate to myoglobin) binding elsewhere (allosteric control)
What is the difference in myoglobin and haemoglobin in structure?
Monomer vs tetramer
What is the difference in myoglobin and haemoglobin in binding curve?
Tighter hyperbolic vs weaker sigmoidal
What is the difference in myoglobin and haemoglobin in function?
Stored in tissue vs transport molecule
What does cooperativity require?
Multiple interacting subunits
What binding curve does cooperativity give?
A sigmoidal curve
What does cooperativity do to binding affinity?
Shifts binding affinity (steep part of binding curve) to a physiologically relevant oxygen concentration
